The present invention relates to an air-fuel ratio control system for an internal combustion engine, which system controls the air-fuel mixture to the stoichiometric air-fuel ratio, at which ratio a three-way catalyst acts most effectively.
In a known air-fuel ratio control system for a motor vehicle, the air-fuel ratio of the air-fuel mixture burned in the engine cylinders is detected as the oxygen concentration in the exhaust gases by means of an O.sub.2 sensor provided in the exhaust system of the engine, and a decision is made dependent on the output signal from the O.sub.2 sensor which indicates whether the air-fuel ratio is richer or leaner than the value corresponding to the stoichiometric air-fuel ratio, for producing a control signal. The control system is provided with a basic pulse generating section for generating basic pulses, and a calculating section which operates to correct the duty ratio of the basic pulses in accordance with the control signal so as to meet driving conditions. The pulses operate an electromagnetic valve so as to control the amount of bleed air in a carburetor for controlling the air-fuel ratio of the mixture. When the duty ratio of the pulses is reduced, the air-fuel mixture is enriched. Thus, the air-fuel ratio is controlled to the stoichiometric air-fuel ratio, at which a three-way catalyst in the exhaust system acts most effectively.
In such an air-fuel ratio control system at cold engine operation, the air-fuel ratio is controlled by open loop control with fixed duty ratios stored in a look-up table. However, the look-up table can not be provided so as to supply an air-fuel mixture having a duty ratio which satisfies both conditions of steady state driving and transient state driving such as acceleration. Although the amount of intake air increases, when the engine is accelerated, the amount of induced fuel does not increase with an increase of the intake air. Accordingly, the air-fuel mixture must be enriched upon acceleration. If the table is made to meet the transient state, the air-fuel ratio is improper for the steady state.